1. Field of the Invention
The present invention relates to a photoelectric conversion device and a method of manufacturing the same. Particularly, the present invention relates to a photoelectric conversion device that uses a substrate on which light receiving pixels including photodiodes and thin film transistors are arranged in an array, and a method of manufacturing the same.
2. Description of Related Art
A photosensor, which is a photoelectric conversion device, is a flat panel that includes a TFT array substrate on which a photodiode that photoelectrically converts visible light into an electric charge and a thin film transistor (TFT) are arranged. The photosensor is widely used in application to a contact image sensor, an X-ray imaging display device or the like. Particularly, a flat panel X-ray imaging display device (which is referred to hereinafter as a flat panel detector (FPD)) in which a scintillator that converts an X-ray into visible light is placed on a TFT array substrate is a promising device for application to the medical industry or the like.
In the X-ray diagnostic imaging field, precision images (still images) or real-time image observation (moving images) are used where appropriate. When taking still images, an X-ray film is mainly used. On the other hand, when taking moving images, an image pickup tube (image intensifier) that combines a photo-multiplier tube and a CCD is used. While the X-ray film has a high spatial resolution, it has a low sensitivity and is capable of taking still images only. Further, the X-ray film requires a processing procedure after taking images and thus has a disadvantage of lack of immediacy. On the other hand, while the image pickup tube has a high sensitivity and is capable of taking moving images, it has a low spatial resolution. Further, the image pickup tube is a vacuum device and thus has a disadvantage of limits on upsizing.
There are two types of FPD: an indirect conversion type that converts an X-ray into light by a scintillator such as CsI and then converts the light into an electric charge by a photodiode, and a direct conversion type that converts an X-ray directly into an electric charge by an X-ray detector element such as Se. The indirect conversion type has a higher quantum efficiency and a better signal-to-noise ratio (S/N ratio), and is capable of visualization and imaging with a lower exposure dose compared to the direct conversion type. A structure and a manufacturing method related to an array substrate of the indirect conversion FPD have been disclosed in Japanese Unexamined Patent Application Publications Nos. 2000-101920 and 2007-165865, for example.
In the array substrate of the FPD, formation of a photodiode that affects the sensitivity of a photosensor, noise and so on is critical. For example, a photodiode in which an electrode made of a transparent conductive film is placed on an amorphous silicon layer, which is a photoelectric conversion layer, placed on an electrode, is disclosed in Japanese Unexamined Patent Application Publication No. 2000-101920. Further, a photodiode in which an insulating film is placed on a semiconductor layer, which is a photoelectric conversion layer, and a metal electrode is directly connected to the semiconductor layer through an opening of the insulating film is disclosed in Japanese Unexamined Patent Application Publication No. 2007-165865.
When forming a photodiode, the photodiode is damaged on its end face due to etching or the like of an amorphous silicon layer, which is a photoelectric conversion layer. Therefore, it is preferred to repair the damage on the end face of the photodiode by performing, after forming the photodiode, processing equal to plasma processing using hydrogen gas (which is referred to hereinafter as H2 plasma processing) that is performed after back-channel etching of an inverse staggered amorphous TFT.
However, in the structure disclosed in Japanese Unexamined Patent Application Publication No. 2000-101920, the electrode placed on the amorphous silicon layer is made of a transparent conductive film, and the transparent conductive film is reduced by damage recovery processing on the end face of the photodiode. Therefore, damage recovery by H2 plasma processing cannot be performed on the photodiode disclosed in Japanese Unexamined Patent Application Publication No. 2000-101920. This causes an increase in leakage current of the photodiode. This is because a leakage path is created in the damaged part of the end face of the photodiode upon application of a reverse bias to the transparent conductive film when using the photodiode as a sensor.
On the other hand, in the photodiode disclosed in Japanese Unexamined Patent Application Publication No. 2007-165865, no transparent conductive film is placed on the semiconductor layer, and therefore damage recovery by H2 plasma processing can be performed. However, in the structure disclosed in Japanese Unexamined Patent Application Publication No. 2007-165865, if the insulating film of a coating type is formed by exposure, a leakage path is created on the end face of the photodiode by an impurity contained in the insulating film. On the other hand, if the insulating film is formed by deposition, not a coating type insulating film, damage is done to the semiconductor layer by dry etching upon making an opening in the insulating film. As a result, rectification at the p-i interface is degraded, which causes an increase in leakage current.
As described above, in the photosensor using the photodiode disclosed in Japanese Unexamined Patent Application Publications Nos. 2000-101920 and 2007-165865, the S/N ratio is deteriorated due to an increase in leakage current. It is thus impossible to obtain a suitable image with a small amount of incident light.
In view of the foregoing, it is desirable to provide a photoelectric conversion device that enables suppression of leakage current of a photodiode, and a method of manufacturing the same.